A large number of polyurethane potting and sealing compositions have been developed and sold commercially over the past several decades. An early discussion regarding mixtures of aromatic diisocyanates and castor oil can be found on pages 788-781 of "Polyurethanes: Chemistry and Technology," Part II, by Saunders and Frisch, Interscience Publishers, John Wiley & Sons, New York (1962). More recent example of aromatic isocyanate based potting and sealing compositions are found in U.S. Pat. Nos. 4,359,359 and 4,224,164 and Canadian Pat. No. 914,850.
U.S. Pat. No. 4,359,359 concerns the method of reacting an aromatic diisocyanate in stoichiometric excess with a mixture of castor oil and trimethylolpropane to yield a prepolymer containing unreacted isocyanate groups and being sufficiently flowable to permit coating onto membrane surfaces, and subsequently hardening the prepolymer by addition of a cross-linking agent consisting essentially of castor oil alone or another mixture of castor oil with trimethylolpropane in an amount sufficient to cross-link a majority of the unreacted isocyanate groups. The addition of a small amount of iron acetyl acetonate is said to improve the cross-linking of the prepolymer and to reduce bubble formation. Further, use of an aromatic diisocyanate which contains about 10 to 50 mole percent isophorone diisocyanate is said to improve adherence to cellulosic membranes. The compositions are said to be useful for mounting semipermeable membranes in dialysis equipment.
U.S. Pat. No. 4,224,164 concerns a potting compound for hollow fiber separatory devices used in biomedical applications. A polyol mixture of at least one polyol and 1 to 15% by weight N-N-N'-N'-tetrakis (2-hydroxypropyl) ethylene diamine (known commercially as "Quadrol", registered trademark of BASF Wyandotte Corp., Parsippany, N.J.) is mixed at an NCO/OH ratio of 1 to 1.1'1 with an isocyanate terminated polyurethane based prepolymer, wherein sufficient Quadrol.RTM. is present to cause the urethane system to increase rapidly in viscosity and thereby reduce wicking of the urethane in the hollow fibers. The polyol may be castor oil, esters of castor oil, polyethers such as polyoxypropylene glycol and polytetramethylene glycol, homopolymers or copolymers of hydroxyl bearing butadiene, and hydroxyl bearing polyesters, such as polyethylene glycol adipate and the like and combinations thereof. The isocyanate may be an aromatic or aliphatic isocyanate. Applicant has tested a composition containing an aliphatic diisocyanate, castor oil, and Quadrol.RTM. and found that it exhibits considerable gassing in the presence of moisture.
Canada Pat. No. 914,850 concerns the condensation products of polyoxyalklyene polyamines and alkylene oxide and a method of making urethane foams with the same. In Example 6 of the Canadian patent a rigid foam is prepared by first combining propoxylated polyoxypropylenetriamine, silicone oil as a surfactant, and trichlorofluoromethane as a blowing agent, and then adding a polyaryl polyisocyanate.
The use of aliphatic isocyanates in potting and sealing compositions is sometimes restricted owing to their significantly higher cost. Their use may be justified where light stability (i.e., absence of darkening on exposure to UV and visible light), higher hydrolytic stability, or the absence of aromatic structures is desired. Aromatic isocyanates are typically prepared from aromatic amines, some of which are potentially toxic or even carcinogenic, and trace amounts of aromatic amines often either persist in the isocyanate or are formed by slow hydrolysis of the isocyanate or the isocyanate derived polymer. The absence of aromatic amines is particularly important in biomedical applications including blood dialysis and in applications involving the treatment of food or potable water.
A problem which frequently arises in the use of either aromatic or aliphatic isocyanate based polyurethane potting compositions with moisture-containing substrates is the tendency of such compositions to gas or form bubbles during the curing cycle. The compositions of the above-referenced patents each have a problem with one or more of bubbling, toxicity, and poor adhesion to cellulosic substrates.